This invention relates to infrared thermal imaging devices and in particular to micro-fabrication of pixelless infrared thermal imaging devices comprising epitaxially integrated quantum well infrared photodetector and light emitting diode.
Infrared imaging is widely used in a variety of applications including night vision, surveillance, search and rescue, remote sensing, and preventive maintenance, to name a few. Imaging devices to provide these applications are typically constructed of HgCdTe or InSb focal plane arrays. These focal plane arrays are known to be pixel mapped devices, where an array element is generally mapped to one or more circuit elements. However, such focal plane arrays are difficult to manufacture and expensive. Quantum Well Infrared Photodetectors (QWIPs) are able to detect Mid to Far InfraRed (M/FIR) light, providing an output current as a result. However, such devices have not been able to be successfully used in efficient and inexpensive image detectors. The basic idea of QWIPs using intraband or intersubband transition for M/FIR detection have been disclosed in U.S. Pat. No. 4,205,331, issued May 27, 1980 to Esaki et al. and in U.S. Pat. No. 4,873,555, issued Oct. 10, 1989, to Coon et al. Embodiments of QWIPs using intraband or intersubband transitions have been disclosed in U.S. Pat. No. 4,894,526, issued Jan. 16, 1990, to Bethea et al. and U.S. Pat. No. 5,023,685, issued Jun. 11, 1991 to Bethea et al. The latter two patents describe a device having improved efficiency by utilizing a series of quantum wells.
An improvement of these earlier technologies was disclosed by one of the present inventors, H. C. Liu, in U.S. Pat. No. 5,567,955, issued Oct. 22, 1996, to the National Research Council of Canada, incorporated herein by reference, wherein the vertical integration of a Light Emitting Diode (LED) with a QWIP is described. The QWIP-LED is a photon frequency up-conversion device. The device comprises either a photo-diode or a photo-conductor connected in series with a LED. The photo-diode or the photo-conductor acts as a M/FIR detector, whereas the LED emits in the NIR or the visible spectrum. A forward constant bias is applied to the LED with respect to the QWIP. A M/FIR excitation of the detector decreases its resistance and thereby increases the voltage dropped across the LED, leading to an increase in the LED emission intensity. Therefore, the incoming M/FIR radiation has been converted into an increase of the NIR or visible emission. The emission in the NIR is efficiently detected by a Si Charge-Couple Device (CCD), resulting in a highly efficient detector. The vertical integration results from epitaxial deposition of the LED material over the QWIP materials.
Details about the QWIP-LED technology as well as numerous embodiments are disclosed in the following references incorporated herein by reference:
U.S. Pat. No. 5,646,421, issued Jul. 8, 1997, to H. C. Liu;
U.S. Pat. No. 6,028,323, issued Feb. 22, 2000, to H. C. Liu;
H. C. Liu, L. B. Allard, M. Buchanan, Z. R. Wasilewski, xe2x80x9cPixelless infrared imaging devicexe2x80x9d, Electronics Letters 33, 5 (1997);
L. B. Allard, H. C. Liu, M. Buchanan, Z. R. Wasilewski, xe2x80x9cPixelless infrared imaging utilizing a p-type quantum well infrared photodetector integrated with a light emitting diodexe2x80x9d, Appl. Phys. Lett. 70, 21 (1997);
E. Dupont, H. C. Liu, M. Buchanan, Z. R. Wasilewski, D. St-Germain, P. Chevrette, xe2x80x9cPixelless infrared imaging devices based on the integration of n-type quantum well infrared photodetector with near-infrared light emitting diodexe2x80x9d, (Photonics West, San Jose, January 1999), SPIE Proc. 3629, 155 (1999);
E. Dupont, H. C. Liu, M. Buchanan, S. Chiu, M. Gao, xe2x80x9cEfficient GaAs light-emitting diods by photon recyclingxe2x80x9d, Appl. Phys. Lett. 76, 4 (2000);
E. Dupont, S. Chiu, xe2x80x9cEfficient light-emitting diodes by photon recycling and their application in pixelless infrared imaging devicesxe2x80x9d, J. Appl. Phys. 87, 1023, (2000);
S. Chiu, M. Buchanan, E. Dupont, C. Py, H. C. Liu, xe2x80x9cSubstrate removal for improved performance of QWIP-LED devices grown on GaAs substratesxe2x80x9d, Infrared Phys. And Techn. 41, 51 (2000); and,
E. Dupont, M. Gao, Z. Wasilewski, H. C. Liu, xe2x80x9cIntegration of n-type and p-type quantum well infrared photodetectors for sequential multicolor operationxe2x80x9d, Appl. Phys. Lett. 78, 14 (2001).
A pixelless thermal imaging device is achieved by a suitably fabricated QWIP-LED having a sufficiently large active area for the detection of a 2-dimensional M/FIR image. The up-conversion device is made sufficiently large in area for sensing a 2-dimensional M/FIR image, and an emitted 2-dimensional image in the NIR or visible spectrum is then detected by a standard Si CCD or other standard imaging device. It is possible to manufacture large format 2-dimensional thermal imaging devices having a perfect fill factor without the need for complex readout circuits. The integrated QWIP-LED technology allows manufacture of efficient and inexpensive thermal imaging devices.
It is, therefore, an object of the invention to provide a micro-fabrication method for manufacturing efficient and inexpensive pixelless infrared thermal imaging devices.
It is further an object of the invention to provide a micro-fabrication method for manufacturing pixelless infrared thermal imaging devices based on epitaxial integration of a QWIP with a LED.
It is yet another object of the invention to provide a micro-fabrication method for manufacturing pixelless infrared thermal imaging devices allowing use of a same manufacturing equipment for producing a large variety of different devices.
The micro-fabrication method according to the invention allows manufacture of numerous different infrared imaging devices based on epitaxial integration of a QWIP with a LED. The various steps of the micro-fabrication method are based on standard manufacturing techniques, for example, epitaxial growth and etching. Furthermore, various different devices are manufactured by changing the order of manufacturing steps, omitting some steps or using different materials. Therefore, it is possible using a same manufacturing equipment for producing a large variety of different imaging devices considerably reducing manufacturing costs.
In accordance with the present invention there is provided a method for micro-fabricating a pixelless thermal imaging device, the imaging device for up-converting a sensed 2-dimensional M/FIR image into a 2-dimensional image in the NIR to visible spectrum in dependence thereupon, the method comprising the steps of:
providing a first substrate, the first substrate having a surface suitable for subsequent crystal growth;
crystallographically growing an integrated QWIP-LED wafer on the surface of the first substrate comprising the steps of:
growing an etch stop layer;
growing a bottom contact layer;
growing a plurality of layers forming a n-type QWIP and a LED; and,
growing a top contact layer;
providing at the top of the QWIP-LED wafer an optical coupler for coupling at least a portion of incident M/FIR light into modes having an electric field component perpendicular to quantum wells of the QWIP;
removing the first substrate; and,
removing the etch stop layer.
In accordance with an aspect of the present invention there is provided a method for micro-fabricating a pixelless thermal imaging device, the imaging device for up-converting a sensed 2-dimensional M/FIR image into a 2-dimensional image in the NIR to visible spectrum in dependence thereupon, the method comprising the steps of:
providing a first substrate, the first substrate having a surface suitable for subsequent crystal growth;
crystallographically growing an integrated QWIP-LED wafer on the surface of the first substrate comprising the steps of:
growing an etch stop layer;
growing a bottom contact layer;
growing a plurality of layers forming a n-type QWIP and a LED; and,
growing a top contact layer;
providing an optical coupler on the top of the QWIP-LED wafer for coupling at least a portion of incident M/FIR light into modes having an electric field component perpendicular to quantum wells of the n-type QWIP;
patterning a device mesa by removing the layers outside the device mesa down to the bottom contact layer, the device mesa approximately comprising an active area of the thermal imaging device, the active area being approximately the size of the 2-dimensional image;
depositing a top metal contact onto the top contact layer such that the top metal contact forms a ring surrounding the active area;
depositing a bottom metal contact onto the bottom contact layer outside the device mesa;
depositing a coating onto the top surface of the active area;
isolating material defects in the active area of the QWIP-LED;
bonding the QWIP-LED wafer to an optical faceplate such that the QWIP-LED is in optical communication with the optical faceplate for light emitted from the LED;
removing the first substrate; and,
removing the etch stop layer.
In accordance with the present invention there is further provided a method for micro-fabricating a pixelless thermal imaging device, the imaging device for up-converting a sensed 2-dimensional M/FIR image into a 2-dimensional image in the NIR to visible spectrum in dependence thereupon, the method comprising the steps of:
providing a first substrate, the first substrate having a surface suitable for subsequent crystal growth;
crystallographically growing on the surface of the first substrate a plurality of layers forming an integrated QWIP-LED wafer;
patterning a device mesa, the device mesa approximately comprising an active area of the thermal imaging device, the active area being approximately the size of the 2-dimensional image; and,
isolating material defects in the active area of the QWIP-LED.
In accordance with another aspect of the present invention there is provided a method for micro-fabricating a pixelless thermal imaging device, the imaging device for up-converting a sensed 2-dimensional M/FIR image into a 2-dimensional image in the NIR to visible spectrum in dependence thereupon, the method comprising the steps of:
providing a first substrate, the first substrate having a surface suitable for subsequent crystal growth;
crystallographically growing an integrated QWIP-LED wafer on the surface of the first substrate comprising the steps of:
growing an etch stop layer;
growing a bottom contact layer;
growing a plurality of layers forming a n-type QWIP and a LED; and,
growing a top contact layer;
patterning a device mesa by removing the layers outside the device mesa down to the bottom contact layer, the device mesa approximately comprising an active area of the thermal imaging device, the active area being approximately the size of the 2-dimensional image;
depositing a top metal contact onto the top contact layer such that the top metal contact forms a ring surrounding the active area;
depositing a bottom metal contact onto the bottom contact layer outside the device mesa;
isolating material defects in the active area of the QWIP-LED;
bonding the top surface of the QWIP-LED wafer to an optical faceplate such that the QWIP-LED is in optical communication with the optical faceplate for light emitted from the LED;
removing the first substrate;
removing the etch stop layer;
providing an optical coupler at the bottom of the QWIP-LED wafer for coupling at least a portion of incident M/FIR light into modes having an electric field component perpendicular to quantum wells of the n-type QWIP; and,
bonding the bottom surface of the QWIP-LED wafer to a plate such that the QWIP-LED is in optical communication with the plate for M/FIR light.